In security critical integrated circuits, some security countermeasures are implemented to provide safety of the critical information against some analysis and attack techniques aimed to obtain the information in an unauthorized way. Active shield is a countermeasure providing security against some attacks depending on physically monitoring or manipulating the integrated circuit from outside. These attack techniques include probing the critical information by making connections to the metal lines of the integrated circuit, faulting the integrated circuit by forcing from these outside connections and changing the connections of the internal metal lines permanently by using FIB (Focused Ion Beam).
In active shield method, the whole surface of the integrated circuit is covered by metal lines on the top metal layer. A transmitter circuitry which supplies a test data to the metal lines covering the whole chip and a number of receiver circuitries which compare the test data received from the top layer metal lines with the original test data received from the transmitter internally are added to integrated circuit. According to the result of the comparison, these receiver circuitries verify the integrity of the top layer metal lines. Since any physical attack will disturb the integrity of the top layer metal lines by making them open or short circuit, the receiver circuitries do not receive the correct test data pattern from the top layer metal lines, thus detect the physical attack.
It is believed that the references; US 2009/0024890 A1, US 2008/0244749 A1, US 2008/0150574 A1, US 2005/0092848 A1 and US 2003/0132777 A1 provide sufficient information on the background of the active shield method. In reference document US 2005/0092848 A1, a way of implementing the active shield method without requiring an additional metal layer is introduced and in reference documents US 2009/0024890 A1 and US 2008/0150574 A1, improvements are aimed to reduce the power consumption due to active shield. The reference US 2008/0244749 A1 introduces some improvements mainly on the detection circuitry part of the active shield method, not on the protection of the top metal layer shield itself.